He was speaking at the first meeting of his cabinet, installed after the overthrow of President Viktor Yanukovych.

The spectre of armed conflict in Crimea will be at the top of a long agenda, the BBC’s Mark Lowen reports from the region.

Heavily armed unidentified soldiers took up position outside the regional parliament in Crimea’s capital, Simferopol, on Saturday. Another airfield in the region was reportedly seized overnight, in addition to two airports and communications centres on Friday.

Ukraine’s interim President, Olexander Turchynov, accused Russia of sending hundreds of soldiers and military aircraft to reinforce its Black Sea Fleet, which is based in Crimea.

Moscow says troop movements on the Crimean peninsula do not infringe international agreements and it denies involvement in the seizing of airports.

Under the agreement governing the Black Sea Fleet, the Russians must coordinate all troop movements outside the fleet’s base areas with the Ukrainian authorities beforehand.

‘Only my orders’

Mr Aksyonov, who leads the main pro-Russian party in Crimea, was elected prime minister of Crimea by the region’s parliament this week in an emergency session, replacing Anatoliy Mohylyov.

In the same vote, the parliament called a referendum on increasing the autonomy of Crimea, a region dominated by ethnic Russians.

Mr Aksyonov’s election has not been approved by the new authorities in Kiev, who traditionally appoint the prime minister of Crimea, in consultation with the regional parliament.

“I appeal to the president of the Russian Federation, Vladimir Putin, to provide assistance in ensuring peace and tranquillity on the territory of the Autonomous Republic of Crimea,” Mr Aksyonov said in a statement.

He went on to announce that he was taking control of security in Crimea “on a temporary basis”.

“All commanders are to obey only my orders and instructions,” Mr Aksyonov said. “I ask all those who refuse to do so to resign.”

‘There will be costs’

Warning Russia there would be “costs” for any military intervention, Mr Obama said he was deeply concerned by reports of Russian military movements.

Speaking from the White House, he commended Ukraine’s interim government for its “restraint”.

“Any violation of Ukraine’s sovereignty and territorial integrity would be deeply destabilising, which is not in the interests of Ukraine, Russia or Europe…” he said.

“The United States will stand with the international community in affirming that there will be costs for any military intervention in Ukraine.”

Mr Obama did not spell out what any US response might be but it could take the form of economic pressure by withholding the deeper trade ties that Moscow seeks.

A G8 summit that Russia is due to host in June might also be boycotted.

The new government in Kiev was formed this week by opposition parties and street activists.

Mr Yatsenyuk said: “The presence of Russian soldiers is a provocation and we demand that Russian soldiers return to their permanent bases.”

“We are taking no steps that could provoke a violent confrontation,” he said. “All responsibility for the escalation of the conflict lies personally at the leadership of the Russian Federation.”

Viktor Yanukovych, who was deposed after months of unrest degenerated into bloodshed this month, made his first appearance in Russia on Friday, insisting he was still the legitimate president of Ukraine and attacking the new government.